Museums and other facilities that keep valuable objects often need security systems that can detect intruders and/or detect tampering with the valuable objects. A variety of systems have been developed to serve these needs. For example, pressure plates can be placed around and/or under the object to detect the weight of an intruder or movement of the object. Other security systems have used webs of light beams and sensors that can detect when an intruder interrupts the web. These security systems often require a secure area that contains at least some system components to prevent an intruder from directly tampering with the system components, even though the object being protected by the web of beams is outside the secure area. Although such systems can be very secure, these security systems are at least theoretically susceptible to being evaded. For example, an intruder may be able to evade a web of light beams by intercepting and replacing security system light beams with light signals that mimic or carry the same information as intercepted light beams.
Use of the same reference symbols in different figures indicates similar or identical items.